A solid oxide fuel cell (SOFC) has attracted attention as clean energy, since SOFC is a fuel cell using a solid electrolyte having oxygen ion conductivity as an electrolyte, whereby an electrochemical reaction to generate electromotive force is an oxidation reaction of hydrogen, and carbon dioxide gas is not generated.
A solid oxide fuel cell usually has a stack structure comprising single cells each comprising a cathode as an oxide, a solid electrolyte and an anode connected by an interconnector. Its operating temperature is usually about 1,000° C., and lowering of the temperature has been attempted and practically employed by various studies, however, it is still a high temperature at a level of at least about 600° C.
As an anode material to constitute an anode, a composite powder (also called as NiO-GDC or NiO-SDC) comprising a NiO powder (NiO phase) and a ceria powder (also called as GDC or SDC) doped with gadolinium or samarium and represented by the formula (I) LnxCe1-xO2-δ (Ln is Gd or Sm) is known as one having an excellent power generation property within the above temperature range.
In general, in the case of an anode comprising GDC or SDC, and NiO, an electron conduction path is formed by an Ni phase formed by the reduction of NiO, an ion conduction path is formed by a GDC phase or an SDC phase, and at 3 type interfaces of the above two phases and a fuel pass for hydrogen, hydrocarbon or the like, an electrode reaction results. Accordingly, in order to improve the electrode property, it is preferred to form a composition in which an NiO phase and a GDC phase or an SDC phase, which are made of the above-mentioned composite powder, are uniformly incorporated (uniform composition) as far as possible at the micro level.
Heretofore, as a method for forming a composite powder (NiO-GDC or NiO-SDC), a method of mechanically mixing an NiO powder and a GDC powder or an SDC powder in a solid state (also called a solid phase method) is widely carried out as the most typical method.
For example, Patent Document 1 discloses an anode for an NiO-ceria type solid oxide fuel cell, and it is described to mix NiO particles, ceria coarse particles having a particle size larger than the NiO particles and ceria fine particles having a particle size smaller than the NiO particles in predetermined proportions to obtain a mixture, whereby the anode material is excellent in gas permeability, and an anode which is excellent in electrode reactivity, conductivity and durability can be formed.
Further, in order to obtain a uniform mixture of particles, a high performance mixer provided with rotating blades to be rotated at a high rate (10,000 rpm) is used. In the case of such a solid phase method, there is a problem such that it is principally difficult to obtain a uniform composition at the micro level, so long as an NiO powder and a ceria powder which are raw material element-containing particles are milled and mixed in solid phase.
Further, Patent Document 2 discloses a process for producing a ceramic powder to be used as an anode material for a solid oxide fuel cell, which comprises a first phase made of nickel oxide (NiO) and a second phase made of ceria doped with a rare earth oxide, and disclosed is a process (also called a complex polymerization method) comprising a step of preparing a raw material liquid containing a metal salt such as Ni (nickel nitrate, cerium nitrate or the like) and a chelating agent having a polymerizable functional group which can chelate a cation such as Ni and a step of forming the raw material liquid into droplets and heating the droplets. In a case where the raw material liquid is heated for polymerization, porous spherical particles are obtained, and in a case where the raw material liquid is not polymerized, hollow particles or laminate particles are formed.
The chelating agent is preferably an oxycarboxylic acid such as citric acid, a polyamine such as ethylenediamine tetraacetic acid or a polyol such as ethylene glycol or propylene glycol.
In the case of this method, the yield is poor, since an intermediate product having a high viscosity is formed by the heat-polymerization, and if a nitrate is used as a raw material, NOx is generated at the time of heating, and thereby an exhaust gas cleaning device is required for carrying out the method on an industrial scale.
Further, according to studies by the present inventors, as shown in the after-mentioned Comparative Example 1, in the case of a citrate method in which a raw material for SDC is simply added in a citric acid aqueous solution, even though a large amount of citric acid is used, the solution becomes a slurry state, and although a mixing state is slightly improved as compared with the solid phase method, there is a problem such that only an insufficient mixing state is realized.